1. Technical Field
The present invention relates to a magnetic fluid seal device for sealing a rotational shaft by using magnetic fluid.
2. Background Art
A magnetic fluid seal device is an optimum device for a bearing mechanism for transmitting the rotational driving force of a motor or the like to a special environment such as an ambient atmosphere having a different outside air pressure difference, a vacuum chamber or the like, for example.
Particularly, the magnetic fluid seal device rotating biaxially on the same axis (hereinafter referred to as “biaxial rotational magnetic fluid seal device”) is broadly applied as a rotational driving part of an X-ray diffraction device for testing semiconductor wafer, a vacuum transfer robot or the like.
FIG. 5 is a side cross-sectional view showing the construction of a biaxial rotational magnetic fluid seal device known as a prior art.
As shown in FIG. 5, the biaxial rotational magnetic fluid seal device 1 is equipped with a cylindrical case 10 which has a bearing hole 2 therein and is made from a non-magnetic material, two shafts being coaxially inserted into the bearing hole 2. Furthermore, it is provided with a cylindrical outer rotational shaft 20 which is freely rotatably supported by a pair of outer bearings 11 provided on the inner peripheral surface of the case 10 and made from a non-magnetic material. Still furthermore, it contains a cylindrical inner rotational shaft 30 which is freely rotatably supported by a pair of inner bearing 21 provided on the inner peripheral surface of the hollow portion of the outer rotational shaft 20, and an intermediate portion 31 of the cylindrical inner rotational shaft 30 is covered with a magnetic material.
The magnetic material is a material having a property that it is easily magnetized under magnetic field caused by magnet, current or the like, and conversely the non-magnetic material is a material having a property that it is hardly magnetized.
An outer magnetic member 12 made from a magnetic material is provided between the case 10 and the outer rotational shaft 20, and likewise an inner magnetic member 22 is also provided between the outer rotational shaft 20 and the inner rotational shaft 30. Furthermore, an outer rotational shaft intermediate portion 28 which covers the outer peripheral surface of the outer rotational shaft 20 and is made from a magnetic material is provided on the confronting surface of the outer magnetic member 12. Magnets 13 and 23 are provided to the intermediate portions of the respective magnetic members 12, 22 under the state that the magnetic pole surfaces thereof face the axial direction. Projecting portions 14, 24 which are triangular in a cross-section are formed on the inner peripheral surfaces of the respective magnetic members 12, 22 so as to be arranged in the axial direction.
Minute gaps are respectively formed between the inner peripheral surface of the outer magnetic member 12 and the outer peripheral surface of the outer rotational shaft intermediate portion 28 and between the inner peripheral surface of the inner magnetic member 22 and the outer peripheral surface of the intermediate portion 31 of the inner rotational shaft 30, and magnetic fluid (not shown) is filed in these minute gaps.
The magnetic fluid is liquid in which ferromagnetic ultrafine particles are stably dispersed, and it is a material having a property that it is magnetized by the effect of the magnetic field of the magnets 13, 23.
The magnetic force occurring from the magnets 13, 23 has a property that it passes along a route having a high magnetic permeability and also it goes to the opposite pole by the shortest distance. Therefore, it magnetizes the magnet peripheral areas of the respective magnetic members 12, 22 and forms a constant magnetic stream. Here, the minute gap portions surrounding the projecting portions 14, 24 provided on the inner peripheral surfaces of the respective magnetic members 12, 22 have small magnetic permeability. Furthermore, the cross-sectional areas of the projecting portions 134, 24 in the magnetic force route are reduced, so that the magnetic densities at the projecting portions 14, 24 are increased. Therefore, the magnetic force concentrates to the projecting portions 14, 24, so that the magnetic stream is attracted to the tips of the projecting portions 14, 24 to magnetize the tips of the projecting portions 14, 24.
As described above, the magnet 13 forms an outer magnetic circuit 15 passing through the magnetized outer magnetic member 12, the magnetic fluid and the outer rotational shaft intermediate portion 28. Furthermore, the magnet 23 forms an inner magnetic member 25 passing through the magnetized inner magnetic member 22, the magnetic fluid and the intermediate portion 31 of the inner rotational shaft 30. As a result, the magnetic fluid is held between the projecting portion 14, 24 and the outer peripheral surface of each rotational shaft 20, 30, and thus the respective rotational shafts 20, 30 can be sealed and supported in a non-contact style by the magnetic fluid.
In the biaxial rotational magnetic fluid seal device, the case and the outer rotational shaft which are made from the non-magnetic material are brought into contact with the outer peripheral surfaces of the outer magnetic member and the inner magnetic member, respectively. The magnetic field caused by the magnet interposed between the respective magnetic members is blocked off by the non-magnetic material, and thus no magnetic circuit is formed in the outer peripheral surface direction of each magnetic member. Accordingly, the magnetic circuit is formed so as to extend from one magnet in only one direction.
In the case of the construction as described above, the magnetic fluid seal device requires individual magnets to form respective magnetic circuits for plural rotational shafts, so that the number of parts is increased, the structure is complicated, and also the device grows in size.